The polyolefins industry continues to look for new and better catalyst systems. Ziegler-Natta catalysts are a mainstay, but single-site (metallocene and non-metallocene) catalysts are making inroads. Among other benefits, single-site catalysts can provide polymers with narrow molecular weight distribution, reduced low molecular weight extractables, and enhanced incorporation of α-olefin comonomers. Traditional metallocenes incorporate one or more cyclopentadienyl (Cp) or Cp-like anionic ligands such as indenyl, fluorenyl, or the like, that donate pi-electrons to a central transition metal. In other non-metallocene single-site catalysts, such as the pyridyl bis(imine) complexes, ligands chelate to the metal through two or more electron donor atoms.
“Clathrochelates” are intriguing compounds in which a transition metal ion is encapsulated within the cavity of a three-dimensional, macropolycyclic ligand. The encapsulated ions, because they are largely sequestered, offer unique properties and chemistry, including interesting redox characteristics and an opportunity to mimic important biological systems. One common type of clathrochelate is an iron(II) tris(dioximate). These are made in one step or sequentially by reacting FeCl2 with three equivalents of a dioxime (nioxime, glyoxime, dimethylglyoxime, or the like) in the presence of two equivalents of a capping reagent (SnBr4 or phenylboronic acid, for example). See, e.g., Polyhedron 17 (1998) 4315.
Despite their obvious complexity, clathrochelates have a high degree of symmetry, and useful synthetic routes have been developed, many by Professor Yan Voloshin and coworkers. A recently published monograph, Voloshin et al., Clathrochelates: Synthesis, Structure and Properties, Elsevier, 2002, provides a comprehensive overview of work in this field and includes hundreds of references to the primary literature.
Voloshin defines clathrochelates as three-dimensional complexes in which an encapsulated metal ion coordinates five or more nitrogen or sulfur donor atoms of an encapsulating ligand that has at least three macrocyclic fragments. The macrocyclic fragments share at least two capping atoms (also called “bridgehead” or “apical” atoms), which are commonly carbon, but can also be transition metals (e.g., Sn, Sb, Mn), silicon, boron, or other atoms. Herein, we expand the definition of “clathrochelate” to include complexes in which the encapsulated metal ion coordinates five or more nitrogen, phosphorus, oxygen, or sulfur atoms.
Clathrochelates have apparently not been suggested as components of catalysts for olefin polymerization. On the other hand, the ability to incorporate transition metals and/or Group 13 atoms in the apical (or capping) positions of clathrochelates provides an as-yet unexplored opportunity to use the unique electronics of clathrochelates for tuning an olefin polymerization reaction.